This invention relates to the field of labeling and identifying objects with an optical label and a corresponding optical identification system.
Photochromic material was used in optical modulator wherein the spatial modulation pattern can be controlled by the intensity pattern of a control light illuminating the modulator. See, U.S. Pat. No. 4,834,511 and U.S. Pat. No. 5,618,654. The photochromic material is incorporated in an etalon type optical cavity. Illumination of the photochromic material by the control light results in a change in the refractive index of the photochromic material. This change in refractive index presumably changes the optical properties of the etalon. The state of the etalon can remain fixed when the programming light is not present. These patents describe transmissive light modulating structures since the etalon produces a peak in the transmission spectrum. Further, the optical etalon can be tuned to a particular wavelength of programming light.
Furthermore, these prior inventions do not provide for a means to selectively and separately encode the modulation response to multiple wavelengths of incident light or a means to select among multiple wavelengths of programming light.
Photochromic layer was incorporated in an optical spatial light modulator. The spatial light modulator makes use of the change in absorption of the photochromic material at the wavelength of a signal light. See, U.S. Pat. No. 6,366,388, herein incorporated by reference. The modulator also contains an optical reflecting filter that selectively reflects the programming light so that the programming light makes two passes through a layer of photochromic material. This reflecting filter for the programming light is located on the side of the photochromic film that is facing away from the direction of the incident programming and signal light.
It is known that optical fiber having a cylindrical shape can be designed to reflect a specific wavelength of light that is incident upon the sides of the fiber. See, S. D. Hart, G. R. Maskaly, B. Temelkuran, P. H. Prideaux, J. D. Joannopoulos and Y. Fink, “External Reflection from Omnidirectional Dielectric Mirror Fibers,” Science, v. 296 (19 Apr. 2002), pp. 510-513; and G. Benoit, K. Kuriki, J. F. Viens, J. D. Joannopoulos and Y. Fink, “Dynamic All-optical Tuning of Transverse Resonant Cavity Modes in Photonic Bandgap Fibers,” Optics Letters, v. 30, n. 13 (2005), pp. 1620-1622, herein incorporated by reference.
The first paper, by Hart, et al., also describes the incorporation of such fiber into a woven fabric. The second paper, Benoit et al., also describes tuning the reflection wavelength by simultaneously applying a light of a visible wavelength to the fiber. This visible wavelength light temporarily changes the wavelength of the optical absorption edge of the chalcogenide material in the fiber in a process known as transient photodarkening. The material that undergoes the transient photodarkening is incorporated in a wavelength selective optical reflecting structure. The accompanying change in the reflection that is due to transient photodarkening persists only while the visible wavelength light is applied on the fiber. To obtain a reversible response, this prior art operates in the transient photodarkening regime which is not associated with a memory effect. Thus, this wavelength selective fiber requires continuous application of a programming light to retain a particular programmed state.
Although it is known that the chalcogenide materials also have a metastable photodarkened state (which might possibly provide a memory effect, although not discussed in the prior art) the approach of Benoit, et al. avoids operation in that regime since the metastable photodarkened condition is not reversible at room temperature. This prior art also does not describe selective and changeable reflection of multiple wavelengths. It also does not provide for a way to select among multiple programming wavelengths.
For the foregoing reasons, there is a need for a programmable wavelength coded optical labeling or identification system wherein the optical label comprises a photochromic material in a multi-layer reflective stack whereby the stack functions as a filter having a reflection peak, not a transmission peak. There is also a need for an optical labeling system wherein the optical label can select among multiple wavelengths of programming and interrogation light. Furthermore, there is a need for an optical labeling system wherein the optical label has a memory function reprogrammable at room temperature at a substantially large standoff distance without making physical contact, wherein the memory does not require sustaining power to maintain the programmed state. There is yet another need for an optical labeling system wherein the coded region may be wrinkled, discontinuous, or folded.